In the field of heat engines, the piston engine has gained wide acceptance in the past century, primarily because of its reliability, controllability, and its desirable power output characteristics. Recently, interest has increased in external combustion piston engines, such as the Stirling cycle and Rankine cycle engines.
Typically, a piston is linked to a mechanism for transforming the reciprocating motion of the piston to smooth rotary motion. Often, this mechanism includes a piston rod extending from the piston to a crank on the crankshaft. The eccentric disposition with respect to the shaft enables it to accommodate the translational motion of the piston rod, and transform the periodic, reciprocating force provided by the piston into smooth rotational motion. Other mechanisms include swash plate and wobble plate devices.
It is well known in the art that the eccentric disposition of the crank of a crankshaft and the motion of a swash plate also causes the piston rod to undergo lateral motion as well as translational motion. This lateral motion is generally accommodated by a wrist pin which links the piston to the respective end of the piston rod. This mechanical system, however, does not entirely alleviate the side thrust which is attendant with the lateral motion of the piston rod. This side thrust, exerted on the piston through the wrist pin, causes uneven wearing of the piston rings, or similar seal, and a degradation of the seal disposed between the piston rod and the end of the cylinder. The side thrust also contributes to the wear of the main bearings which support the crank shaft.
Although the wear which is associated with the side thrust of the piston rod has been minimized by superior design, balancing, and machining, it still remains a significant factor in limiting the life span of an engine. The end of this lifespan is often determined by the time at which the engine must be rebuilt, including new piston rings, seals, bearings, and the like.